Robin Williams was explaining the main theme of his new film, "Patch Adams," which opens nationally on Christmas. In the movie, he plays an outrageous, older-than-usual medical school student who believes that the power of laughter and caring can have very real health benefits.

In the late '60s, the real-life Adams was depressed and suicidal. Committing himself to a mental institution in Virginia, he discovered he had a gift to help others. So he decided to go to medical school, where he was at odds with the establishment, which saw his antics to brighten patients' lives as being unprofessional. (The real Patch Adams eventually formed the Gesundheit Institute, dedicated to a more connected, personalized approach to medicine.)

"There is such a thing as bedside manner," says Williams during a recent weekend of interviews promoting the film in Manhattan. "And some medical schools have gone back to teach that." Suddenly he turns from sensitive to silly. "I don't often touch in interviews, you know. Let's hug! Oh, daddy!"

Robin Williams reaches out and touches his interviewer gently on the hand.

"Just this can be very reassuring," he says, gently caressing the palm.

Williams was explaining the main theme of his new film, "Patch Adams," which opens nationally on Christmas. In the movie, he plays an outrageous, older-than-usual medical school student who believes that the power of laughter and caring can have very real health benefits.

In the late '60s, the real-life Adams was depressed and suicidal. Committing himself to a mental institution in Virginia, he discovered he had a gift to help others. So he decided to go to medical school, where he was at odds with the establishment, which saw his antics to brighten patients' lives as being unprofessional. (The real Patch Adams eventually formed the Gesundheit Institute, dedicated to a more connected, personalized approach to medicine.)

"There is such a thing as bedside manner," says Williams during a recent weekend of interviews promoting the film in Manhattan. "And some medical schools have gone back to teach that." Suddenly he turns from sensitive to silly. "I don't often touch in interviews, you know. Let's hug! Oh, daddy!"

It's hard not to laugh. And feel good.

Williams has personally seen the power of laughter, of caring, of the human touch on patients, ranging from children from the Make a Wish Foundation, to his good friend Christopher Reeve, who was paralyzed in a horse-riding accident.

"It's all about connections," he says.

And science has proved him right, with research proving that laughter and caring increases endorphins -- naturally occurring painkilling substances in the human body -- which reduces the amount of medication patents require and speeds their rate of recovery.

And the benefits are for the giver as well as the patient.

"For me, laughter is cheaper than Prozac," Williams says, half seriously. when I see a patient or a child or Chris laugh, I get it back. It's it's symbiotic."

But Williams is not so big a fool as to advocate laughter and caring as the only prescription for illness.

"It's a very delicate thing to know when is the right time {to bring in the humor} to a patient," he says. "With Chris, there are certain things I can joke about, and other moments when I know it's not appropriate and just to be there for him."

And like the best humor, timing is everything. He understands doctors can't always be silly or overly sensitive.

"There has to be a certain professional quality to a doctor, a certain clinical distance," he says. "You can't have a heart surgeon in the middle of an operation go: `No! Arrrggghhhhh!! I can't cut it!!!'"

But Williams himself has never been in such a state of depression where humor wasn't welcome, even for a cameo appearance.

"I was raised by a mother who was a Christian Scientist," he says. "I call her a Christian Dior Scientist. She is basically optimistic, which is frightening sometimes. On the other hand, I was also raised by a father who was a veteran who had been blown across a bridge in a kamikaze attack and who was left to die. So he gave me one view and she gave me another, and between the two of them basically you function. And that's how I lived. He was very much a realist about his life. And she took the optimistic road where everything was wonderful and blueberries and happiness. And between the two of them, they named me Carolina - no! They gave me both sides of the deal."

In the film, Patch Adams engages his patients by asking what excites them. How would he answer that question?

"Creation," he says without skipping a beat. "To create ideas, to really find an idea that has a resonance. It involves performing, but it's also about connecting to people and bouncing ideas off other people and creating things together."

He says that at age 46 and as the father of three children (his oldest son is 16), he finds himself more focused. "Before it was like this scattergun wild stuff, and now all of a sudden it's different," he says.

Williams still drops by unannounced at comedy clubs, but the visits are more about exploring his humor in a more relaxed way than going gonzo with off-the-wall zaniness that is sure to get the yucks.

"Like last night, I didn't have an idea what I was going to talk about," he says. "And it was nice to try to go deeper and go a little darker. Let's see what we can talk about. It was weird. We just started talking and it was interesting, though it wasn't necessarily funny. But people were going, `Whoa, we're taking a different turn.' And I thought, `Better come back. Better dance again.' It was interesting."

Busy Year

Williams has been prolific of late, with "Good Will Hunting" getting wider release early this year, followed by "When Dreams May Come" and now "Patch Adams." He has already filmed "Jakob the Liar," which takes place in the Jewish ghetto in Poland in 1944 and is scheduled to be released next fall. ("That couldn't come out this year because it would have been just too many films," he says. "They'd start advertising movies without me. But I do worry about being overexposed.")

Williams says he may be having an easy year in 1999 if "Andrew Martin" -- a film based on an Isaac Asimov sci-fi story, "Bicentennial Man," about a robot who works for one family for 200 years -- does not happen.

"I'll just stay home or do some live performing," he says, ruling out a return to theater. (He performed in "Waiting for Godot" with Steve Martin, directed by Mike Nichols, in the late '80s.)

With "Patch Adams," it's the third time Williams has played a sweet and sympathetic doctor, following his roles in "Awakenings," where he played Dr. Oliver Sacks, and in "Good Will Hunting," in which he played a gentle psychiatrist -- and won an Oscar.

In the future, he says, he'd love to play "a real son of a bitch."

But casting Williams in such a role "is not something that comes to people's minds immediately, so I have to go out and look for these parts myself."

He says he learned that the one villainous role he did a few years ago, "Joseph Conrad's The Secret Agent," was a favorite of Unabomber Theodore Kaczynski, "who is someone you don't want as a fan, especially if you get mail."